The present invention relates to a statics switch for providing multiple current paths between a power source and a load that is energized by that source. More specifically the invention relates to a static switch and switch controller for efficient switching between active current carrying devices in a way less likely to cause damage to circuits connected to the static switch.
It is often important and sometimes critical that a back up power source be available in case a primary power source is either unavailable or degrades until it is not suitable for powering a load. A hospital or a large computer center may, for example, have access to two separate sources of alternating current power for operating some or all of the equipment.
A prior art switched power source is disclosed in U.S. Pat. No. 5,138,184 to Keefe. This patent discloses a solid state, static power supply control system for transferring a load from a preferred source of AC power to an alternate source of AC power.
Cyberex, Inc., assignee of the present invention, is one of a number of suppliers of solid state switches. These switches utilize pairs of gate activated silicon controlled rectifiers or SCRs that are connected in parallel. Each pair of SCRs conducts current from a single pole of alternating current to a load. One SCR conducts current in one direction and a second, oppositely connected SCR conducts current in a second direction as the AC source switches polarity. When both SCRs are gated into conduction, first one and then the other SCR provides a low resistance path for alternating current power as current flow alternates back and forth during the AC power cycle.
A transfer from a faulty power source to an alternate power source requires the active or conductive SCRs be de-activated and a second set of SCRs pairs be activated to couple an alternate power source to the load. Such a transfer should be accomplished with a minimum disruption of current flow to the load and also should be accomplished with no current transfer between the two power sources.
Present gate control algorithms generally use a technique that suffers certain deficiencies. Upon initiation of a transfer command, a first set of SCRs that are carrying current are ungated. When current stops flowing through this first set of SCRs, a second set of SCRs through which current is to begin passing are gated on. For multiple pole switches, either individual poles are treated separately, or entire banks of poles associated with the ports are first ungated and then a new bank gated on.
Prior art methods guarantee current distribution during the switching process. Current disruption for one half of a 60 hertz power signal is not important for most applications but can be critical in data monitoring and processing operations. Present switching methods result in particularly severe disruption of current when three phase power sources are not exactly aligned in phase, or do not share a common neutral connection.
In the copending application to Galm there is described in algorithm for controlling the switching from an active set of SCRs that are conducting to a second inactive set of SCRs that are rendered conductive in a specified manner. One feature of the invention disclosed in the Galm application is the confirmation of the sense of current flow through a pair of parallel connected SCRs subsequent to an initial determination of the current flow through those SCRs.
While a preferred use of the invention disclosed in the Galm application works well under almost all conditions, certain loads have caused switching that makes the algorithm less efficient that it should be. The present invention addresses those instances of inappropriate switching by use of a somewhat different criteria for determining the state of a conducting set of SCRs.
Earlier filed U.S. patent application 08/412,067 discloses a method and apparatus for providing alternate current paths to a junction through different sets of gate controlled current carrying devices coupled to the junction. The preferred method disclosed in this application starts with the step of initiating a switch from an active device set to an inactive device set by removing a gate signal from two parallel path, opposite sense current carrying devices which in combination form the active device set. Such a step might be initiated for example in response to an indication that one alternating current source was faulty so that a switch would be instituted to connect a load to a secondary source. Experience with the system disclosed in this application indicates that for certain loads (typically inductive loads) the switching process is not conducted as efficiently as for other loads and the present invention addresses such situations.
Apparatus constructed in accordance with one embodiment of the present invention couples either a source or a load to a junction by controlled activation of first and second sets of gate controlled switches that are coupled to the junction. A monitoring circuit monitors a current status of the switches of the first and second sets of gate controlled switches by categorizing a current status of the devices that make up a device set as having either a positive current carrying state; a negative current carrying state; or no current site, i.e. in a blocking state.
The apparatus provides alternate current paths to a junction through gate controlled circuit carrying devices such as silicon controlled rectifiers. A switch is initiated away from a first current path through a first, active device set of two or more current carrying devices to a second current path through a second, inactive device set of two or more current carrying devices by removing a current sustaining gate signal from the current carrying devices of the first device set.
A current status of the devices of the first, active device set is determined by monitoring a voltage across the first device set. If the voltage falls within a specified range at least one of the devices that make up the active device set is designated as being current carrying and otherwise designated as blocking current in both directions. The voltage across the devices of the first device set is again sensed to confirm the current carrying status of the devices.
If the current carrying state of the devices in the first device set is confirmed to be in the same state and the device set is not blocking current in both directions, one or more gate controlled device in a second, inactive set of said gate controlled devices are gated to enable current flow in one direction. If the sensing of voltages indicates the first set is blocking, then devices in the second, inactive device set can be gated to enable current flow in both directions.
The present invention has applications in single phase as well as multiple phase alternating current power systems. From the above it is apparent that one object of the invention is method and apparatus for controlling the switching of a gate controlled switching system to effectively and safely accomplish that switching. These and other objects, advantages and features of the present invention will be better understood by reference to a preferred embodiment of the present invention which is described in conjunction with the accompanying drawings.